1. Field of the Invention
The present invention relates to liquid level indicators operating electronically using a capacitive sensor array. More particularly, the present invention relates to a capacitive liquid level indicator having capacitive sensors and associated circuitry for providing very high sensitivity to detecting the amount thereof submerged in a liquid.
2. Description of the Prior Art
Knowledge of the level of a liquid in a container provides valuable information for a number of reasons. Not only does this information provide fullness of a tank and volume of liquid data (based upon the dimensions of the tank), but this information also provides operational data regarding ancillary devices which are dependent upon the presence of liquid in the tank. An example of such a situation is a vehicle fuel tank.
Devices to measure liquid levels generally fall into two types of liquid level indicators. A first (and older) type of liquid level indicator relies upon fluid mechanics, whereby a pivotable arm has a distally connected float which buoyantly interacts with a surface portion of the liquid, the angle of the pivot arm providing an indication of the liquid level. A second (and newer) type of liquid level indicator relies upon electrical properties of the liquid with respect to which sensors submerged in the liquid generate signals to an electronic circuit, which, in turn, provides indication of the level of the liquid. In view of the potential for increased accuracy of electronic liquid level indication (as opposed to mechanical liquid level indication), the art has struggled with providing reliable and accurate electronic liquid level indicators.
With regard to electronic liquid level indicators, the electrical property of choice for sensing liquid level is the dielectric property of the liquid as compared to that of air above the liquid. In order to sense the dielectric property of the liquid, capacitive sensors are used, wherein the electrical field between the plates is affected by the presence of the liquid, the dielectric constant of which being greater than air.
Examples of capacitive liquid level indicating devices are U.S. Pat. Nos. 5,406,843; 5,144,835; 5,142,909; 5,103,368; 5,051,921; 4,624,139; 4,603,581; 4,594893; 4,564,881; and 4,553,434. Generally speaking, capacitive based liquid level indicating devices are composed of an array of capacitive sensors, or a capacitive sensor in the form of a single set of plates (fingers), arranged vertically in the liquid, as well as an electronic circuit for processing signals from the capacitive sensor(s) responsive to changes in capacitance thereof due to changes in the area thereof in contact with the liquid, and for actuating a level indicator, such as a fuel gauge.
What remains a problem in the art of capacitive liquid level indicators are spurious capacitive values introduced by the capacitance of the leads between the capacitive sensors and the electronic circuit which lead capacitance mixes with the sensor capacitance, thereby adversely affecting the ability of the electronic circuit to accurately indicate the liquid level under all conditions.
What remains yet another problem in the art of capacitive liquid level indicators are parasitic electric fields associated with the dielectric substrate to which the capacitive sensors are affixed. The term "parasitic electric fields" herein means any electric field inside the substrate, in contradistinction to electric fields outside the substrate which are referred to herein as "sensing electric fields". In this regard, there are two kinds of parasitic electric fields: that which passes through the substrate between the plates (ie., fingers) of a capacitive sensor, referred to herein as a "cross-plate parasitic electric field" and that which passes generally through the substrate, referred to herein as "trans-substrate parasitic electric fields". Parasitic electric fields can adversely affect the sensitivity of a capacitive sensor to detecting submersion in liquid if the parasitic electric fields are relatively large in relation to the sensor electric field.
Parasitic electric fields have been addressed in the art with only limited success, such as for example by the presence of a ground plate as shown at FIG. 1. In this regard, a capacitive liquid film indicator 10 has a dielectric substrate 12 which supports on one side an interdigitated capacitive sensor 14. Successive plates in the form of fingers 16 of the capacitive sensor 14 have opposite polarity (fingers having a positive polarity are designated by a "+"and fingers having a negative polarity are designated by a "-"), wherein the number of fingers is optional On an opposite side of the substrate 12 is a ground plate 20 which is tied to ground. However, as depicted at FIG. 1, the ground plate 20 is believed to interact with the capacitive sensor 14, whereby a trans-substrate parasitic field 22a is present through the substrate 12, in addition to the cross-fingers parasitic electric field 22b. The sensing electric field 24 of the capacitive sensor 14, which is external to the substrate 12 is believed to be not so much larger than the parasitic electric fields 22a, 22b that the influence of the parasitic electric fields, particularly the trans-substrate parasitic electric field, can be ignored. Indeed, under certain operational conditions, it is believed that an erroneous liquid level indication may possibly be generated. For a discussion of the issues surrounding this capacitive sensor technology, see U.S. Pat. No. 5,175,505.
What yet further remains another problem in the art of capacitive liquid level indicators is lack of ability to inherently accommodate a range of liquid types without resort to a reference capacitive sensor. This problem is related to the limited sensitivity range of the capacitive sensors when arranged as an array on a dielectric substrate. In this regard, the prior art typically teaches a reference capacitive sensor located in the liquid for biasing the capacitive sensor array to the dielectric constant of the liquid. Problematical with this concept is if the reference capacitive sensor should become exposed to air due to low liquid levels in the tank, the capacitive array will bias to the dielectric constant of air, and then generate a signal indicating a full tank when the tank is, in fact, empty.
Accordingly, what is needed in the art is a capacitive liquid level indicator which has sufficiently low lead capacitance that lead capacitance can be ignored, has sufficiently low parasitic electric fields that these fields can be ignored, and has a sufficiently wide sensitivity range so as to accommodate a wide range of liquid types without need of a reference capacitive sensor.